1. Field of the Invention:
The invention relates to epitaxial layers of semiconductor materials and somewhat more particularly to a method and apparatus for producing expitaxial layers via liquid phase shift epitaxy.
2. Prior Art:
Processes for producing epitaxial layers of semiconductor material, such as composed of A.sub.III B.sub.V -compounds, on monocrystalline substrates via liquid phase shift epitaxy are known wherein a surface of a substrate is contacted with a solution of such compounds and out of which, as a result of oversaturation, an epitaxial layer is deposited on the substrate surface and the solution is then removed from the substrate and the overlying epitaxial layer.
For example, German Offenlegungsschrift No. 26 41 347 describes a liquid phase shift epitaxy process of this type and a device for practicing such process.
Normally, the liquid phase shift epitaxy process involves a strong growth at the edges of the epitaxial layer, which is generally referred to as "bead-growth". However, when a plurality of layer sequences are produced in one operation, certain disadvantages occur because of such bead-growth. One disadvantage is that residues of a removed solution remain suspended on the beads and during the next application of a solution, which can be different from the first removed solution, are drawn into such subsequent solution and contaminate it. Another disadvantage is that portions of the beads break-off when the solution chamber is passed over the newly deposited epitaxial layer and thus mechanically damage such epitaxial layer.
These disadvantages make it nearly impossible to produce quality semiconductor components, such as for laser diodes. A further disadvantage resulting from the presence of beads is that instances when the substrate and overlying epitaxial layer are further processed via planar techniques, a lack of definition occurs during focusing in the photographic process.
A plurality of methods for avoiding bead-growth at the edges of a deposited epitaxial layer or substrate are known. For example, German Offenlegungsschrift 24 04 017 describes a process wherein, prior to the epitaxial process, the edge zones of the substrate are covered with a masking layer composed of a heat-resistant material (silicon dioxide or silicon nitride).
Western Electric Technical Digest, No. 46, page 77 describes another means of avoiding bead-growth whereby the chambers containing the melt solution are additionally heated by heating coils positioned thereabout in order to minimize and/or eliminate heat discharge through the graphite chamber walls.
The Journal Cryst. Growth, Vol. 29, (1975), pages 62-64 describes yet another means of avoiding bead-growth whereby a so-called step-cooling process is used during the epitaxial deposition.
Finally, the earlier mentioned German Offenlegungsschrift No. 26 41 347 suggests that bead formation at the substrate edges can be avoided by modifying the spatial orientation of the substrate edges, based on the fact that the speed of crystal growth at edges is dependent upon the spatial orientation of such edges. In this process, substrates are selected which are split and sawn in such a manner that the boundary edges of the substrate surfaces are not edges at which rapid bead-growth typically occurs.